The present invention relates to the field of systems for recovering organic solvent vapors such as toluene and benzene. The recovery systems in prior art processes involve chilling a gas stream containing organic solvent vapors to condense the vapors for recovery purposes, without freezing the vapors.
One prior art approach involves spraying liquid nitrogen into nitrogen gas to cool the gas to a predetermined temperature wherein the gas is then employed as an intermediate cooling fluid used to chill and condense solvent vapors in a conventional heat exchanger. A blower is emloyed to circulate the gas. This approach is relatively expensive, makes inefficient use of the liquid nitrogen refrigerant, and is potentially subject to the problems of high solvent viscosity and trace water freezing to be discussed below.
Another system is similar to the above mentioned prior art approach, but uses gas buoyancy effects to circulate the gas without the use of the blower. The equipment required is large, relatively expensive, and potentially subject to high solvent viscosity and trace water freezing problems.
Another approach involves passing liquid nitrogen through a heat exchanger also containing an intermediate fluid which freezes below the boiling point of nitrogen. The intermediate fluid is thereafter involved in direct heat exchange with the organic solvent vapor stream. This system is relatively expensive and is also subject to high solvent viscosity and trace water freezing problems.
The inventor commenced constructing a system employing a tower having packing within an intermediate portion and a liquid reservoir of recovered solvent contained within the lower portion of the tower. Solvent vapor within a stream of nitrogen was introduced below the packing and flowed upwardly through the packing. The liquid solvent was drawn from the reservoir through an indirect heat exchange within which it was chilled, by liquid nitrogen boiling within the exchanger, and then returned to the top of the tower. The chilled liquid solvent flowed downwardly through the packing, chilling the rising vapor and condensing out most of the solvent which accumulated within the reservoir. This system may be characterized as a kind of scrubber. The heat exchanger was a "Trane" aluminum core unit with several layers of dummy core between the liquid nitrogen containing tubing and the solvent containing tuning. Liquid nitrogen was introduced into the heat exchanger and had a temperature of about -320.degree. F. However, typical solvents freeze around -50.degree. to -100.degree. F. and ideally, one wishes to chill the solvent very close to its freezing point without freezing it, in order to recover as much solvent as possible by condensation in the intermediate portion of the tower containing the packing.
A control valve is employed to control and limit the flow rate, and hence the temperature of the boiling nitrogen in the exchanger in response to a signal from a temperature transducer. If the liquid nitrogen in the exchanger is too cold, liquid solvent will freeze in the exchanger and the system is fouled, and must be shut down. If it is not cold enough, the resulting warmer liquid solvent transported to the packing will cause less solvent to be condensed than would otherwise ideally be the case. Thus, maintaining the temperature of the liquid nitrogen between narrow limits within the heat exchanger is highly desirable and yet is somewhat difficult to achieve.
Another problem encountered by the inventor was that the liquid solvents often became so viscous at the resulting low temperatures that they became difficult to pump through the tiny tubing passages of the exchanger. The result is the requirement for additional wasteful pumping power and a larger, more expensive pump.
The inventor also tested this system for sensitivity to water in the vapor stream. About 2% water vapor was added to the incoming gaseous nitrogen and solvent vapor, and all surfaces in the scrubber were wet with liquid solvent. It was hoped that any water entering would freeze into tiny ice crystals immediately upon exposure to the below zero scrubber. The tiny ice crystals were suspended in the liquid solvent and did not cause clogging or fouling of passages in the heat exchanger. However, when the system was turned off, the ice melted and when the system was thereafter started up, the water froze in the heat exchanger and clogged its passages. As a result, the system would have to be completely drained every time it was turned off, and the system would have to thereafter be refilled with water-free solvent before start-up. This procedure is a serious operating nuisance.
It is object of the present invention to eliminate the heat exchanger described above in order to solve the above stated problems in connection with high solvent viscosity and the freezing of residual water upon startup.
It is a further object of the present invention to eliminate the necessity of maintaining the solvent just above its freezing point without actually freezing the solvent in the heat exchanger which results in clogging of the passages therein.
It is yet a further object of the invention to reduce costs by eliminating the heat exchanger altogether.